I was under the impression that stretching nylon within the limits of its elastic properties does not weaken it. The nylon rebounds with no (or no appreciable) weakening. It also rebounds to the original diameter once stress is removed. Stretching nylon outside the boundaries of its elastic properties (into the plastic zone) will certainly weaken it.
Yes and no. It all depends on what you expect.
Nylon absorbs water over time; with fishing line this is usually several minutes. Once saturated, nylon is MUCH limper and more controllable. It's also, BTW, at a greater volume. (That's why some people spool line, go fishing for 10 minutes, and then notice that they've got a weird sound. The spool of line has expanded, hitting the foot or crossbar.) And yes, wet nylon is weaker than dry nylon. The percentage varies, and I've never seen it matter to me with line over 12 lb. test.
So you never HAVE to stretch it. Instead, you CHOOSE to stretch it. And nothing is "wrong".
One of the reasons nothing is wrong is that manufacturers formulate nylon with varying properties. Some is stiff and wiry (but abrasion resistant) and some is soft and supple. The soft and supple stuff won't need stretching or KVD unless you're picky. And some people are picky, and you might be one of them. Nothing wrong with that.
Generalizations don't usually cover both picky and non-picky people. Horses for courses.
And to throw another monkey wrench into the discussion, you need to remember that heat gives dry nylon a set. So if you lay a rod 'n reel out in the hot sun in the middle of August, you're gonna have a lot more problems.
Try this: get a little spray bottle. Put a drop of Dawn dishwashing soap in it. Fill it with water. Just before you start fishing, spray your spool of line. Soak it.
See if that doesn't make a big difference.
25 years ago, I used Stren original. I always stretched it. I use Stren Original now, too. The formulation has changed, and I never have any problems with it now. Things change.
Good luck fishing! jj